The practice of regional anesthesia (the administration of anesthesia to a specific body region) is not new. Today, increasing numbers of patients are receiving nerve blocks for anesthetic purposes during surgery and for extended relief of trauma or chronic pain.
Successful administration of regional anesthesia depends primarily on the accurate placement of anesthesia in relation to the target nerve. For sensory nerves, accuracy of placement is determined by paresthesia (a buzzing or tingling sensation) reported by the patient to the physician. Obviously, the success rate of a nerve block will be low when the patient gives an inaccurate report of paresthesia or when the patient is disoriented, sedated or otherwise not fully functional. Equally critical to the success of a nerve block is the skill or experience of the anesthesiologist attempting to localize a nerve.
In an effort to increase the success rate of a nerve block, some anesthesiologists x-ray patients before administering anesthetic solution to determine the exact location of the anesthesia needle vis-a-vis the target nerve. Although somewhat helpful, this technique proves to be impractical, expensive, and not always readily available.
More recently, peripheral nerve stimulators have been put into practice as a means of effectively locating peripheral nerves. Nerve localization via electrical stimulation is based on the fact that an electrical pulse can stimulate a nerve fiber to contract an innervated muscle or cause paresthesia in the case of sensory nerve stimulation.
Over the years, nerve stimulators have taken the form of insulated (or uninsulated) anesthesia needles connected to a source of electricity. To localize a nerve, the electrified anesthesia needle is placed within the tissue of the body in the vicinity of the nerve to be blocked. The needle is then used as a stimulating probe until stimulation of the target nerve is achieved as determined by visually detecting muscle contractions or by eliciting a report that the patient feels the stimulus. The current supplied by the electrical source is reduced while the anesthetist simultaneously advances or redirects the needle within the tissue until nerve stimulation is obtained using a lower amperage current. An injection of a portion of the anesthetic dose is then administered to the patient to terminate the response of the nerve to the electrical pulse. If the nerve response is terminated, the anesthesia needle is deemed to be in the vicinity of the target nerve (often with actual needle-to-nerve contact), and the remaining dose of anesthetic is administered to the patient. This same technique is employed regardless of whether the nerve to be localized is motor or sensory. A description of this nerve localization technique is discussed in greater detail in Raj et al., "Use of the Nerve Stimulator for Peripheral Blocks", Regional Anesthesia, April-June 1980, pp. 14-21.
Examples of nerve stimulators for assisting in the administration of anesthesia may be found in U.S. Pat. No. 3,682,162 to Coyler and U.S. Pat. No. 4,515,168 to Chester et al. The Coyler patent generally discloses a combined electrode and syringe needle which acts as a stimulation probe when the syringe needle is connected to an electrical supply.
The Chester et al. patent discloses a nerve stimulator which is clamped onto the syringe of a conventional syringe and anesthesia needle assembly. The unit contains a power supply, a pulse generating circuit, and a manually controlled current-adjusting potentiometer which allows the operator to adjust the current supplied to the needle.
Although both of the above-described devices are effective in stimulating a peripheral nerve, errors in administering anesthesia to obtain a nerve block may be encountered. This occurs because the success of the nerve block is dependent upon the cooperation of the patient and the skill of the anesthesiologist. For example, when stimulating sensory nerves, the anesthesiologist must rely on the patient's ability to perceive and to describe the degree of tingling for information regarding the effectiveness of electrical nerve stimulation. Thus, if the patient is unable to communicate or accurately evaluate paresthesia, anesthesia may be delivered to an improper location resulting in an ineffective nerve block or an overdose of anesthesia. On the other hand, when stimulating a motor nerve, the anesthesiologist must pay close attention to the associated muscle to avoid missing any contraction of the muscle or other anatomical cue indicative of needle location relative to the target nerve. Furthermore, regardless of whether the nerve to be localized is motor or sensory, the anesthesiologist must manually adjust the strength of the electrical current. This manual adjustment requires an assistant who is not "scrubbed" (i.e. whose hands do not need to remain sterile and who can therefore handle the control knob, which is not sterilized) thereby leaving room for human error which could, in extreme cases, cause infection by contamination and/or permanently damage the peripheral nerve. Thus, within the field of regional anesthesia, a need exists for a nerve stimulator which overcomes the weaknesses of the aforementioned devices and effectively localizes peripheral nerves for anesthetic purposes.
Nerves are localized for other, non-anesthetic purposes and an electrical nerve stimulator (although used in a different manner) can be useful for these purposes as well. For example, during surgical procedures, the operating surgeon must avoid cutting nerves which are essential for specific motor or sensory functions. To avoid cutting such nerves, the surgeon may use a nerve stimulator to determine the exact location of a nerve to guard against inadvertent cutting.
An example of a nerve stimulator used for this purpose is disclosed in U.S. Pat. No. 2,704,064 to Fizzell et al. The Fizzell et al. patent discloses a neuromuscular stimulator having two probes for passing a current to a subcutaneous nerve. The probes are placed on the body in the area of the nerve to be stimulated and an operating surgeon watches for a response to the applied current. If a response to the current is observed, the surgeon avoids cutting in that particular area to prevent inadvertent severing of a nerve. Thus, by probing carefully, the surgeon may excise tumorous tissue, for example, without destroying nerves essential for specific body functions.
The Fizzell et al. device suffers from some of the same disadvantages as the nerve stimulators used for regional anesthesia, including manual adjustment of electrical current and close observation of associated muscles by the operating surgeon or an assistant and inability to localize autonomic nerves or visceral somatic nerves. Thus, within the field of surgery there also exists a need for a nerve stimulator which localizes and guards peripheral nerves easily and effectively.